1. Field of the Invention
This invention relates to an apparatus for recording information by scanning a recording medium with light beam converged into a spot-like shape and, in particular, to an optical information recording apparatus provided with focusing and/or tracking servo means for the light beam.
2. Related Background Art
As examples of a medium using light to record information thereon and read out the recorded information, there are known various forms such as a disc-like form, a card-like form, and a tape-like form. Among them, an optical information recording medium formed into a card-like shape (hereinafter referred to as an "optical card") has a great estimated demand as a medium having a large recording capacity which is compact and light in weight and convenient to carry.
Referring to FIG. 1 showing a schematic plan view of such an optical card 101, reference numeral 102 denotes an information recording area; 103 indicates information tracks; 104 and 104' denote track selecting areas; and 105 denotes a home position of a light beam spot.
On the optical card 101, information is optically detectably recorded as a record bit row (information tracks) by scanning the optical card with a light beam modulated in accordance with recording information and is reduced down into a minute spot. At that time, to record the information accurately without causing any trouble such as intersection between the information tracks, it is necessary to control the irradiating position of the light beam spot on the surface of the optical card in the direction perpendicular to the scanning direction (the lengthwise direction of the tracks) (auto tracking which will hereinafter be referred to as "AT"). It is also necessary to control the light beam in the direction perpendicular to the surface of the optical card (auto focusing which will hereinafter be referred to as "AF") to irradiate the light beam as a minute spot of a predetermined size in spite of any bending or mechanical error of the optical card. Also, the above AT and AF are necessary during reproduction.
Referring now, to FIG. 2 showing a construction of an apparatus, for recording information on and, for reproducing information from the optical card, reference numeral 106 denotes a motor for driving the optical card 101 in the direction of the double-head arrow parallel to the lengthwise direction of the tracks; 107 indicates a light source such as a semiconductor laser; 108 a collimating lens for collimating the light from the light source 107; 109 denotes a beam splitter; 110 an objective lens; 111 a coil for tracking; 112 a coil for focusing; 113 and 114 denote condenser lenses; 115 and 116 photoelectric conversion elements; 117 denotes a tracking control circuit; and 118 a focusing control circuit. Electric current is supplied to the coil 111 for tracking and the coil 112 for focusing by commands from the control circuits 117 and 118 on the basis of a tracking signal and a focusing signal detected by the photoelectric conversion elements 115 and 116 to thereby move the objective lens 110 and to accomplish the AT and AF. Reference numeral 119 designates a system controller for controlling the recording/reproducing apparatus, and reference numeral 120 denotes a group of various control signals output from the system controller. Other signals than the signals 120 are also output from the controller 119, but they are not shown. Reference numeral 121 designates an optical head, and reference numeral 122 denotes a drive motor for moving the optical head in the direction of arrow u in FIG. 1.
In accordance with an information signal which is input from a terminal 123, a modulation circuit 124 generates a signal modulated between at least two levels which are not zero and outputs the modulated signal to the light source 107. The light source 107 emits a light beam intensity modulated between two levels which are not zero in accordance with the modulated signal.
The light from the light source 107 is collimated by the collimating lens 108 and passes through the beam splitter 109 and, thereafter, is converged onto a recording track on the optical card 101 by the objective lens 110. The light reflected by the recording track is now transmitted through the beam splitter 109 and is divided into two light beams by the beam splitter 109. The divided light beams are converged onto the tracking signal detecting photoelectric conversion element 115 and the focusing signal detecting photoelectric conversion element 116 by the condenser lenses 113 and 114, respectively. Signals obtained by the photoelectric conversion elements 115 and 116 are transformed to a tracking error signal and a focusing error signal by the tracking control circuit 117 and the focusing control circuit 118, respectively. By energizing the tracking coil 111 and the focusing coil 112, the objective lens 110 is moved to thereby execute the AT and AF.
FIG. 3 is a detailed diagram of the tracking control circuit 117.
In FIG. 3, reference numeral 301 designates reflected light resulting from the light from the light source which was reflected by the medium, reference numerals 115-A and 115-B denote photoelectric transducers divided into two by a dividing line extending in a direction corresponding to the lengthwise direction of the tracks for detecting a tracking error, and reference numeral 207 designates a subtraction circuit for subtracting signals D and E which are signals from the photoelectric transducers 115-A and 115-B and for outputting a tracking error signal. Reference numeral 206 denotes an adder circuit for adding the signals D and E; 203 designates a dividing circuit for dividing an output G of the subtraction circuit 207 and an output F of the adder circuit 206; 202 a phase compensator circuit for stabilizing the AT servo; 204 a driver for supplying a drive current to the objective lens; and 205 a tracking controller for receiving the signal 120 from the system controller 119 and controlling the entire tracking control circuit 117.
FIGS. 4A through 4D are timing charts of the signals at various positions in the circuit of FIG. 3 during recording.
FIG. 4A shows a recording information signal, FIG. 4B shows a variation in power P of the light source modulated by the above signal, FIG. 4C shows a variation in voltages of the tracking signals D and E obtained by the power of the light source via the photoelectric transducers 115-A and 115-B, and FIG. 4D shows a variation in voltage of the addition signal F of the tracking signals D and E.
Based on signals "1" and "0" of the recording information the power P of the light source varies between into two levels P.sub.H and P.sub.L When the power is at the P.sub.H level, a pit is formed and when the power is at the P.sub.L level, no pit is formed. At this time, the signals D and E also vary between two levels. Therefore, when these signals D and E are only subtracted, the open loop gain of the AT servo when the power of the light source is set to P.sub.H is P.sub.H /P.sub.L times as large as the open loop gain of the AT servo when the power of the light source is set to P.sub.L. Thus, a problem such that oscillation easily occurs and the servo becomes unstable are caused. Particularly, in the case of an optical card in which the linear velocity is low and the zone of recording information is near the zone of the AT servo, the component of the recording information is mixed with the AT servo. As a result, the objective lens is unnecessarily moved, so that stable AT servo cannot be performed. Consequently, as an example of a method of making the open loop gain of the AT servo constant in both cases when the power of the light source is set to P.sub.L and when the power of the light source is set to P.sub.H, there is a method using the dividing circuit 203 as shown in FIG. 3. That is, the output G of the subtraction circuit 207 is input to the numerator side input terminal Y of the dividing circuit 203 and the output signal F of the adder circuit 206 is input to the denominator side input terminal X of the dividing circuit 203. Thus, the variation by recording modulation is offset because even if the signals D and E vary between V.sub.L and V.sub.H as shown in FIG. 4C the signal F likewise varies between V.sub.DL and V.sub.DH as shown in FIG. 4D. Therefore, the open loop gain of the AT servo can always be made constant and stable AT servo operation can be realized.
The above method is also applicable to the AF servo. For example, a focusing signal is detected by using an anamorphic optical system like a toric lens as the lens 114 in FIG. 2 and by using as the photoelectric conversion element 116 an element whose light-receiving surface is divided into four portions 116-A through 116-D as shown in FIG. 5. A spot converged onto the light-receiving surface by the lens 114 is circular when the light beam is focused onto the medium, but when out-of-focus occurs, the spot is deformed in conformity with the direction of the out-of-focus as indicated by a broken line and an alternate long and short dash line in FIG. 5. Accordingly, a sum signal O of outputs of the two portions 116-A and 116-C which exist at the diagonal positions and a sum signal P of outputs of the two portions 116-B and 116-D are derived by adders 130 and 131, respectively. The sum signals O and P are differentiated by a differential amplifier 132 to thereby obtain a focusing signal.
The basic construction of the optical card recording-reproducing apparatus as describe above has been disclosed, for example, in U.S. Pat. No. 4,912,697. Also, the dividing, circuit of the focusing servo has been described in Japanese Laid-Open Patent Application No. 52-134704.
However, the dividing circuit 203 in the above-mentioned example of the related background art effects complicated analog processes therein and, therefore, is expensive and lacks precision. Thus, the servo circuit becomes expensive as a whole and a disadvantage arises such that the accuracy of the information recording during recording deteriorates.
On the other hand, an optical information recording/reproducing apparatus in which the gain of the focusing servo is switched between the period of time for recording and the period of time for reproduction during which the intensity of the light beam irradiated to the medium differs has been disclosed in Japanese Laid-Open Patent Application No. 52-80802. However, such an apparatus also presents a problem such that during the recording the gain is always constant and, particularly, at a point of time when the intensity of the light beam varies, the servo becomes unstable.
To solve the problems in Japanese Laid-Open Patent Application, No. 52-80802 mentioned above, in copending U.S. patent application Ser. No. 07/794,644, further, the same applicant as the present invention has already proposed an optical information recording apparatus having means for sampling and holding a servo signal during the period of time when the intensity of the light beam is being shifted between different levels. However, in such an apparatus, when a signal to drive a light source changes from a higher level to a lower level, although the gain of an amplifier is instantaneously switched from the low level to the high level, a slight delay time exists until the intensity of the light beam is changed to the low level. Therefore, a signal corresponding to the light beam of the high intensity is input to the high gain amplifier and an output of the amplifier is saturated, so that ringing occurs in the output waveform of the amplifier for the period of time until the output is returned to the normal state. It is considered that the ringing makes the tracking/focusing servo unstable.